Immunization of pigs with Actinobacillus pleuropneumonia live attenuated (gene-deleted) vaccine HB04M intramuscularly or intranasally exhibits remarkably rapid protection against heterologous strain challenge.

Actinobacillus pleuropneumonia (APP) is the etiological agent of porcine pleuropneumonia, characterized by acute hemorrhagic fibrinous pleuropneumonia and chronic fibrinous necrotizing pleuropneumonia. The acute manifestation of APP is marked by a high fatality rate, leading to substantial economic repercussions to the global swine industry. Current vaccination strategies against APP primarily include bacterin vaccines, subunit vaccines, and live attenuated vaccines. However, their immediate protective efficacy after immunization, particularly for emergency scenarios, remains poorly characterized. In this study, pigs were immunized with APP live attenuated (gene-deleted) vaccine HB04M (serovar 7) via intramuscular or intranasal route at 3 and 7 days prior to challenge with a virulent heterologous serotype strain (serovar 5) at a lethal dose. The findings revealed that pigs receiving intramuscular vaccination 7 days pre-challenge demonstrated a significantly effective immunological defense, achieving a 100% survival rate with minimal lung injury. Intramuscular vaccination 3 days and intranasal vaccination 7 days pre-challenge both exhibited 80% protection, while intranasal vaccination 3 days pre-challenge offered only 60% protection against the challenge. The immediate protection observed 3 days post-immunization was correlated with the rapid vaccine-induced IFN-γ response, while protection at 7 days post-immunization was enhanced by the synergistic effects of HB04M-induced antibodies and IFN-γ. Overall, HB04M demonstrated significant protection against a lethal dose of the heterologous strain as early as 3 days post-immunization, with intramuscular vaccination delivering nearly complete protection by 7 days post-immunization. These findings suggest that HB04M could serve as an effective emergency vaccination strategy during APP outbreaks in pig farms, providing timely protection to mitigate morbidity and mortality.